RESEARCH PAPER

The April 24, 2013 Changning Ms4.8 earthquake: a feltearthquake that occurred in Paleozoic sediment

Xiangfang Zeng • Libo Han • Yaolin Shi

Received: 27 September 2013 / Accepted: 2 January 2014 / Published online: 22 January 2014

� The Seismological Society of China, Institute of Geophysics, China Earthquake Administration and Springer-Verlag Berlin Heidelberg 2014

Abstract The dense broadband seismic network provides

more high-quality waveform that is helpful to improve

constraint focal depth of shallow earthquake. Many shal-

low earthquakes occurring in sediment were regarded as

induced events. In Sichuan basin, gas industry and salt

mining are dependent on fluid injection technique that

triggers microseismicity. We adopted waveform inversion

method with regional records to obtain focal mechanism of

an Ms4.8 earthquake at Changning. The result suggested

that the Changning earthquake occurred at a ESE thrust

fault, and its focal depth was about 3 km. The depth phases

including teleseismic pP phase and regional sPL phase

shows that the focal depth is about 2 km. The strong, short-

period surface wave suggests that this event is a very

shallow earthquake. The amplitude ratio between Rayleigh

wave and direct S wave was also used to estimate the

source depth of the mainshock. The focal depth (2–4 km) is

far less than the depth of the sedimentary layer thickness

(6–8 km) in epicentral region. It is close to the depth of

fluid injection of salt mining, which may imply that this

event was triggered by the industrial activity.

Keywords Induced earthquake � Depth phase �Waveform inversion

1 Introduction

An earthquake sequence including the Ms4.8 mainshock

(Changning earthquake for short) shocked Changning,

Sichuan, on April 25, 2013, which injured dozens and

resulted in economic losses amounting to about 300 million

RMB (Fig. 1). The Chinese Earthquake Network Center

reported that both the Ms4.8 mainshock and the strongest

aftershock occurred at a depth of 4 km. (http://www.cenc.

ac.cn/manage/html/402881891275f6df011275f971990001/

__SUBAO/_content/13_04/25/13e3e2158b900.html), while

the USGS National Earthquake Information Center deter-

minated the mainshock is a M5.3 event occurring at 10-km

depth (http://comcat.cr.usgs.gov/earthquakes/eventpage/usb000

gfau#summary). Such divergence increases more interest in

improving accuracy of focal parameters.

Most deconstructive and felt earthquakes in Sichuan

occur at major fault systems such as the Longmenshan

Fault system. However, the epicenter of Changning earth-

quake is about 40 km away from the Huayingshan Fault

that is a major fault in eastern Sichuan basin. Since the

thickness of sediment in southern Sichuan depression

ranges from 6 to 8 km (Song and Luo 1995), the Chang-

ning earthquake was eventually an earthquake occurring in

sediment. Because the strength of rock in shallow crust is

too weak to accumulate enough strain, most earthquakes

occur in the middle crust except in the area with anomaly

geothermal flow (Shi and Zhu 2003; Klose and Seeber

2007). Therefore, there are only a few articles about

deconstructive earthquakes occurring in shallow crust

(\5 km) in India and Austria cratons (Dawson et al. 2008;

Gupta et al. 1996). Luo et al. (2011) reported a decon-

structive event occurring in Mesozoic sediment at the

center of Sichuan basin. The 2010 Christchurch earthquake

is also another recent example (Kaiser et al. 2012). Such

X. Zeng � Y. Shi (&)

CAS Key Laboratory of Computational Geodynamics,

University of Chinese Academy of Sciences, Beijing 100049,

China

e-mail: shiyl@ucas.ac.cn

L. Han

Institute of Geophysics, China Earthquake Administration,

Beijing 100081, China

123

Earthq Sci (2014) 27(1):107–115

DOI 10.1007/s11589-014-0062-3

cases provide an opportunity to study strain accumulation

and failure in weak rock.

In recent years, production of shale gas experienced a

great boom in the US, and many countries proposed

ambitious plans for shale gas exploration and production.

Because the permeability of shale or other tight rocks is

very low, engineers pump huge volumes of pressurized mix

of water, chemicals, and sand to create and hold open

fractures. Seismicity events induced by fluid injection in

geothermal sites (e.g., Eberhart-Phillips and Oppenheimer

1984) and wastewater disposal sites (e.g., Healy et al.

1968) have been reported in the recent decades. Several

moderate and felt earthquakes (up to Mw5.7, Keranen et al.

2013) occurred in shale-gas fields in the US midcontinent,

and such a situation was proposed to be posing a higher

risk (Ellsworth 2013). There are several articles on induced

seismicity in gas field (Zhu et al. 2007; Long et al. 2010)

and salt mine (Lu et al. 2009). Changning–Weiyuan field is

the first shale-gas production area of China. The historical

seismicity concentrates in the range of 10–20-km depth.

Therefore, the Changning earthquake is a rare case study to

analyze seismic hazard caused by shallow event.

Due to sparsity of the sample, uncertainties of source

parameters of shallow earthquake determinated with

104˚36' 104˚48' 105˚00' 105˚12'

28˚00'

28˚12'

28˚24'

28˚36'

28˚48'

10 km 0 10 20 30

20130424_Ms4.8

200608012006092220070814 20071005

Fig. 1 Historical seismicity (M [ 2.0) between 2009 and 2013 at Changning. The red star denotes location of salt mine

108 Earthq Sci (2014) 27(1):107–115

123

traditional methods are sizable. For travel-time location

except nearby stations, sparse samples of takeoff vector in

upper hemisphere increase uncertainty introduced by

tradeoff between depth and origin time (Mori 1991). For

first motion inversion, uncertainties of the depth and

velocity model also introduce large errors. Methods based

on waveform could use more information taken from later

phases to constrain fault plane and centroid depth. In this

article, we adopted waveform inversion method to obtain

focal mechanism including centroid depth, and then used

local and teleseismic depth phases and amplitude ratio of

body wave and surface wave to determinate the focal

depth.

2 Focal mechanism

The cut and paste (CAP for short) method is a popular

waveform inversion method, which uses regional three-

component records of body wave and surface wave to

constrain fault plane and centroid depth (Zhao and Helm-

berger 1994; Zhu and Helmberger 1996). Recently, the

teleseismic waveform was also introduced into CAP to

provide more constraint for thrust event (Ni et al. 2010;

Chen et al. 2012). We collected three-component records at

stations within 200 km (Fig. 2a; Zheng et al. 2009) and

adopted Sichuan basin velocity model obtained from short-

period surface-wave dispersion inversion (Fig. 2b; Xie

et al. 2012). Velocity increases with the depth in sediment,

and the total thickness of the crust reaches 40 km. Syn-

thetic seismograms of arbitrary faults are built from those

of the three basic faults that were computed with FK

method (Zhu and Rivera 2002). Both synthetic and

observed waveforms were bandpass filtered (0.02–0.15 Hz

for body wave and 0.02–0.1 Hz for surface wave). Grid

search scheme was employed to seek the best solutions at

different depths, and the misfit function reaches the mini-

mum at 3 km as the best centroid depth (3 km; Fig. 3a),

while the magnitude of moment is 4.5. The fault planes of

the best solution are 128�/42�/83� (plane I) and 317�/48�/96� (plane II) for strike, dip, and rake, respectively. Fig-

ure 3b shows comparison between the filtered synthetic

and observed waveforms. Most cross-correlation coeffi-

cients of Pnl Segment are larger than 0.8 except CQT and

WAS stations that are further away from the epicenter. For

surface-wave segment, the synthetic waveforms best fit

observations at ROC and YAJ stations, and the cross-cor-

relation coefficients are larger than 0.7. Strike of fault plane

I is consistent with the Changning Anticline, and it is close

to fault planes of historical events obtained from the

amplitude ratio method (Ruan et al. 2008). Consequently,

the fault plane I is possibly the rupture plane.

(a) (b)

Fig. 2 a Epicenteral and seismic stations used in this study. Star denotes epicenter, while triangles show stations. Faults are shown in solid lines,

b basin velocity model. Dashed line denotes shear wave, while Vp is shown in solid line

Earthq Sci (2014) 27(1):107–115 109

123

3 Depth phases

The depth phases are reflected at free surface upper

hypocenter, and the paths of depth phase and reference

phase are close to each other except the reflected segment.

Hence, the differential time between two phases is domi-

nated by depth, and it is slightly affected by heterogeneity

along propagation path. Benefiting from radiation pattern

of thrust event, the global seismic network provides high-

quality records of the Changning earthquake. We selected

broadband records at Global Seismographic Network.

After removing instrument response and lowpass filtering

(\1 Hz), we chose three high signal–noise-ratio records at

different azimuths (Fig. 4a). When the focal depth is small,

the pP and sP signals may be contaminated by direct P

wave, and manual picking is difficult. The waveform

modeling method, which best fits observation with syn-

thetic waveforms of different focal depths, is a better

choice. The synthetic waveform was computed with three

steps involving effects of mantle, source, and receiver-side

crust (Kikuchi and Kanamori 1982). The source-side crust

model is the same as the previous one, and the velocity

model in the mantle is that of PREM model. The t* (1.0 for

P wave) was adopted to take into account the effect of

inelastic attenuation. Figure 4b shows the comparison

between the filtered observations and the synthetic wave-

forms of different focal depths. All traces were filtered and

aligned with direct P arrival. The differential time between

P and pP shows a clear growth trend in synthetic wave-

forms, and the preferred depth is around 1–2 km.

We also analyzed depth phases as recorded by the

regional network. The sPL phase is an effective one for

determining focal depth at near distance, and it has been

widely used in several moderate earthquake studies (Luo

et al. 2010; Chong et al. 2010). The dominant frequency of

sPL is lower than direct P wave, and the radial component

is much stronger than the vertical one. Three-component

records of JLI pertain to the removed instrument response,

and velocity records were integrated into displacement.

Both the synthetic and observed waveforms are bandpass

filtered between 0.05 and 1.0 Hz. As Fig. 5a shows, the

sPL signal at the radial component is much stronger than

the one at the vertical component. The synthetic wave-

forms of 2 and 3 km fit the observation better than other

4.5

4.4

4.4

4.54.5

4.5

4.5

4.5

4.5

4.5

300

350

400

450

500

550

Mis

mat

ch

0 2 4 6 8 10Depth (km)

CQ.CQT

2.0583

2.0580

1.2589

8.0085

CQ.ROC

1.0596

1.0596

0.8097

0.8092

0.8099

CQ.WAS

2.1081

0.7595

0.7581

2.0098

GZ.BJT

-1.4089

-1.4090

-1.9598

SC.HWS

0.5583

0.5589

1.7096

1.4586

SC.JYA

1.2585

1.2590

2.8095

SC.LBO

-1.7584

-1.7588

-0.3096

-0.3089

-1.9597

SC.LZH

0.6597

0.6599

0.0598

0.0591

SC.MGU

-0.9589

0.5094

0.5079

-1.0597

Pnl Z Pnl R Surface Z Surface R Surface T

YN.YAJ

-1.2584

-1.2597

0.0098

0.0090

-1.3596

YN.ZAT

-1.1094

0.1591

0.1589

(a)

(b)

Fig. 3 a Waveform misfit versus focal depth, b waveform comparison between synthetic (red) and observed waveforms (black line)

110 Earthq Sci (2014) 27(1):107–115

123

ANTO

COLA

TIXI

0

2

4

6

8

10

z (k

m)

0 4 8 12Time (s)

0 4 8 12Time (s)

0 4 8 12Time (s)

(a)

(b)

ANTO COLA TIXI

pP pP pP

Fig. 4 a Teleseismic stations map, b waveform comparison between the teleseismic synthetic (red) and the observed (black) ones

Earthq Sci (2014) 27(1):107–115 111

123

ones. The sPL is also clear for records of Ms4.2 aftershock,

and the differential time between sPL and P suggests that

depth of this aftershock is close to that of the mainshock

(Fig. 5b).

4 Short-period surface wave

At regional networks, we observed strong short-period

surface wave that is an index of shallow earthquake. Tsai

and Aki (1970) proposed that spectrum of surface wave

could be used to constrain focal depth with the well-con-

strained fault plane. The short-period Rg wave also is

considered as an important characteristic of shallow event

(Kafka, 1990; Luo et al. 2011). Both the 90-degree phase

differences between radial and vertical components and

dispersion are helpful to us to identify Rg wave. Figure 6

shows clear Rg wave signals at HWS and LZH stations. In

general, the Rg wave could be observed when epicentral

distance exceeds by about five times of focal depth (Luo

et al. 2011). Consequently, the Rg wave at HWS (dis-

tance = 31 km) suggests that the focal depth of the

Changning earthquake may be not larger than 6 km. The

amplitude ratio of the body wave and the surface wave is

also sensitive to the focal depth (Luo et al. 2011). We

compare the different ratios at the synthetic waveform and

observation, as shown in Fig. 6. As Fig. 6 shows, deeper

focal depth results in weaker Rg wave. At HWS, the best

depth is about 2–4 km, while the other one is about 3–5 km

at LZH. Most of the energy of the short-period Rg wave

was trapped in sediment where inelastic attenuation is

much stronger than that at middle crust. While path of the

body wave bends toward the middle crust, inelastic atten-

uation of the body wave slightly increases with the

0

2

4

6

8

10

Z (

km)

0 4 8Time (s)

0

2

4

6

8

10

Z (

km)

0 4 8Time (s)

(a) (b)

sPL sPL

Fig. 5 a sPL waveform fitting at JLI station for Ms4.8 mainshock, b sPL waveform fitting at JLI station for Ms4.2 aftershock. Red the synthetic

vertical component: gray the synthetic radial component; and blue the observed radial component

112 Earthq Sci (2014) 27(1):107–115

123

distance. Therefore, amplitude ratio between Rg and body

wave at farther stations will be smaller than the one mea-

sured at closer station.

5 Discussion and conclusions

As mentioned above, the Changning earthquake is a thrust

event on ESE fault plane, while theestimated focal depth is

between 2 and 4 km. The preferred depth of 3 km was

obtained from CAP inversion, and it is supported by both

depth phases and short-period surface observations. In the

southern Sichuan depression, thicknesses of Paleozoic and

Mesozoic sediments are about 7 km (Song and Luo 1995),

and so the hypocenter is positioned in the Paleozoic

Dengying Formation. The most-deconstructive earthquake

occurs in the crystalline basement, and Horton et al. (2005)

reported a rare case of a Mw4.2 earthquake that occurred in

Paleozoic sediment in Kentucky, U.S.A. The number of

fellable earthquake at Changning has been significantly

increasing since 2006 (Ruan et al. 2008). In October 2007,

Earthquake Administration of Sichuan Province installed

two temporal broadband seismometers to monitor micro-

seismicity. The location result shows that most small

earthquakes are shallower than 3 km, while stronger events

occurred at greater depths, while the centroid of earth-

quakes between October 18, 2007 and November 12, 2007

lies at about 10 km east of the salt mine. The semi-major

axis of the earthquake cluster is close to that of the fault

plane I of this study’s result. The focal mechanism solution

indicates the axis of the compressive principal stress along

NE. This result is similar to the previous result (Ruan et al.

2008), but it is different from the result of the regional

tectonic stress field (Zhu et al. 2007). Mckenzie (1969)

0

2

4

6

8

10

Z (

km)

0 10 20 30Time (s)

0

2

4

6

8

10

Z (

km)

10 20 30 40

Time (s)

(a) (b)

RgS wave RgS wave

Fig. 6 a Observed (black) and synthetic (red) short-period surface waves at HWS, b observed (black) and synthetic (red) short-period surface

waves at LZH

Earthq Sci (2014) 27(1):107–115 113

123

pointed out that the stress axes obtained from a few

earthquakes might deviate from that of the regional tec-

tonic stress. Another potential interpretation is that the

present tectonic stress is different from the one in geo-

logical time. The rupture fault was formed in different

tectonic stresses, and a strong horizontal stress induced the

thrust sliding.

Changning–Weiyuan region is one among the first pilot

shale-gas fields of China. Since hydraulic fracking

increases concern on seismic hazard, the Changning

earthquake provides a case to study the mechanism of

earthquake in sediment that will be helpful to us to

understand induced seismicity. The salt mining is a his-

torical industry in Sichuan basin, and previous studies

proposed that fluid injection during the salt mining induced

microseismicity (e.g., Lu et al. 2009; Long et al. 2010). At

Changning, the salt deposit is about 2,400–2,900 m deep,

and the deepest injection well is about 3,000 m (Ruan et al.

2008). The number of felt earthquake shows similar vari-

ation with the net injected fluid volume (Ruan et al. 2008).

Injected fluid not only induces seismicity around well but

also possibly diffuses to nearby fault and results in the

increase of pore pressure. The induced seismicity in

Arkansas shows that the fluid injection increases the

potential of deconstructive earthquake at blind faults

(Horton 2012) even tens of kilometers away. Flows in the

lower crust and the upper mantle may enhance stress in the

upper crust, and it leads to the seismogenic fault reaching

critical state (Zoback and Townend 2001). The lower

mantle flow beneath Sichuan basin has been reported in

recent decades, and high-frequency-induced seismicity in

Rongchang region also supported the fact that faults are in

critical state. Therefore, even a small stress perturbation

caused by injected fluid could induce earthquake. Induced

seismicity such as that occurred in the Changning earth-

quake raises public concern on seismic hazard.

Since the Changning earthquake occurred only 5 days

after the April 20, Lushan Ms7.0 earthquake and separation

between two earthquakes is \300 km, the Changning

earthquake raised concern on whether the Lushan earth-

quake would trigger strong earthquake in adjacent region.

There are two main potential mechanisms about earthquake

trigger: static and dynamic triggering mechnisms. Coseis-

mis static displacement changes strain at nearby faults, and

the resulting effect could be described as change of Cou-

lomb failure stress (DCFS). Positive DCFS means that risk

of earthquake increases, and the DCFS threshold of trig-

gering is about 0.1 MPa (King et al. 1994). The coseismic

static displacement decreases with distance as 1/r–1/r2. For

example, in the Mw7.3 Landers earthquake, the DCFS is

only about 0.003 MPa at 200 km far away (Hill et al.

1993). Shan et al. (2013) show that DCFS caused by the

Lushan earthquake at Huayingshan fault is less than

-0.01 MPa. Therefore, the possibility of static trigger is

very low. The amplitude of surface wave excited by large

earthquake decreases with distance, as such displacement

could induce strong stress perturbation. Therefore, the

dynamic triggering is still significant at far field. For

example, the maximum dynamic stress perturbation caused

by the Mw7.9 Denali earthquake is up to 0.12 MPa at

3,000 km far away (Pankow et al. 2004). Most dynamic

triggered events occurred in a few minutes after arrival of

surface wave. However, Brodsky and Prejean (2005) pro-

posed the fluid migration is a potential mechanism of

dynamic trigger, and delay time would be affected by

permeability and diffusion coefficient (Glowacka et al.

2002). The delay time ranges from seconds to days (Mo-

hamad et al. 2000; Hough 2005). After fluid injection in

past decades, rock in sediment may have been saturated

and easier to be triggered. However, there is not clear

creditable clue of triggering.

In summary, focal mechanism of the Changning earth-

quake was obtained from waveform inversion, and the fault

plane I (128�/42�/83�) is possibly the rupture plane. The

focal depth (2–4 km) was determinated with the depth

phases and amplitude ratio of body wave and surface wave.

Such shallow earthquake in Paleozoic sediment is possibly

induced by fluid injection rather than triggered by the

Lushan earthquake.

Acknowledgments The authors thank Dr. Townend and other two

anonymous reviewers for their constructive comments. This work was

supported by China National Special Fund for Earthquake Scientific

Research in Public Interest (201308013), China Postdoctoral Science

Foundation (No. 2012M520431), and the National Natural Science

Foundation of China Grant No. 41204044.

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